Two isoforms of arginase, A1 and A2, were found in rat liver, submaxillary gland and kidney as well as beef kidney. In beef liver, however, A2 was the only detectable form. Two additional forms, A3 and A4 , found only in rat kidney were probably artifactitious. A1 and A2 exhibited chromatographic and immunological microheterogeneity. While A1 in rat liver and submaxillary gland was excluded by DEAE-cellulose (pH 8.3) and retained on CM-cellulose (pH 7.5), that (A’1) in beef and rat kidneys was excluded by both ion-exchangers. A2in all tissues was retained on DEAE-cellulose, but not on CM-cellulose. Both A 1and A2 in rat liver and beef kidney, A1 from rat submaxillary gland and A2 from beef liver were precipitated by antibodies to rat and beef liver arginases. None of the forms in rat kidney (A1, A2 , A3 and A4 ) showed any cross-reactivity to either antibody. Rat submaxillary gland A2 was precipitated by anti-rat liver arginase, but activated by anti-beef liver arginase. While the major molecular forms were A1 in rat liver and submaxillary gland and A2 in beef liver and rat kidney, the two forms occurred in equal proportions in beef kidney. It appears that different isoforms might function as components of the urea cycle in the liver of different mammals and of the arginine catabolic pathway in different extrahepatic tissues.

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